A conventional organic dispersion electro luminescence (EL) element typically includes a luminescent layer, which is formed by dispersing electroluminescent phosphor in dielectric, and two electrodes respectively disposed at both sides of the luminescent layer so that the organic dispersion EL panel emits light when an AC voltage is applied between the electrodes. At least one of the electrodes is a transparent electrode. Such an organic dispersion EL panel is typically used in a back light of a cell phone or other displays.
A typical example of electroluminescent phosphors used in organic dispersion EL panels of this type is a zinc sulfide phosphor using zinc sulfide (ZnS) as a host material, which contains an activator and a co-activator. The activator contains either one of or both copper (Cu) or manganese (Mn). The co-activator contains at least one element selected from among chlorine (Cl), bromine (Br), iodine (I), and aluminum (Al).
Various methods have conventionally been offered to improve luminous efficiency, life span, and other various characteristics of electroluminescent phosphor of the zinc sulfide type. One of the known examples calls for adding an activator and a co-activating and particle growth-accelerating agent (flux) to a host material, which is made of zinc sulfide or the like, eliminating unformed particles by adding a non-halide magnesium compound during the firing process, and evening out the size and shape of particles. One concrete example of such methods is described in Patent Reference Document 1, which refers to a method of forming electroluminescent phosphor by applying a primary firing treatment to a mixture consisting of a host material, to which an activator and a co-activating and particle growth-accelerating agent (flux) have been added, and subjecting the mixture to a secondary firing. This method characteristically calls for adding a non-halide magnesium compound to the mixture during either one of or both the primary firing and the secondary firing so that the molar ratio of the non-halide magnesium compound is in the range of 0.001 to 0.1, i.e. 0.1 to 10 mol %, to 1 mol of the host material.
Patent Reference Document 1
Japanese Laid-open Patent Publication No. 1996-90262 (Page 3, FIG. 1)
The description of the method of producing electroluminescent phosphor in the reference document mentioned above refers to magnesium sulfate as an example of a non-halide magnesium compound. The description also refers to magnesium oxide and magnesium nitrate as other examples of suitable compounds. In the case of the method described above, however, the non-halide magnesium compound is used merely to prevent generation of unformed particles or not-fully-formed particles so as to even out the size and shape of particles. There is a reference in the above patent document to the effectiveness of adding a magnesium compound during the secondary firing process. However, as the secondary firing is performed at a temperature ranging from 700 to 750° C., it is difficult for solid chemical reaction to progress between the zinc sulfide and the magnesium compound, which may be magnesium sulfate.
For the reasons as above, the aforementioned electroluminescent phosphor production method is unable to produce electroluminescent phosphor having luminance and life span that meet the demands of the users for a wide range of uses for the electroluminescent phosphor. It is therefore necessary to improve the luminance and life span of electroluminescent phosphor produced by a conventional method, such as the one described above.
In order to solve the above problems, an object of the present invention is to provide electroluminescent phosphor having superior luminance and a long life.